This invention relates to a filament wound frame for a game racquet such as a tennis racquet, racquetball racquet, squash racquet, badminton racquet, etc. More particularly, the invention relates to a game racquet frame which is formed by filament winding a tow of fibers and thermoplastic material.
All thermoplastic racquets currently in production are made of layers of braided sleeves of commingled thermoplastic filaments and reinforcing fibers. These 2 dimensional braided sleeves are layered to make a racquet preform and then heated in a mold to produce a game racquet frame. The braiding process adds to the cost of producing a preform, is limited to relatively high wind angles, and produces a bulky, woven pattern that buckles the load-carrying fibers, thereby reducing compressive and shear strengths.
U.S. Pat. No. 5,176,868 describes making a frame from a tow of fibers, for example carbon fibers, and thermoplastic material. The tow can be formed from commingled or co-wound graphite fibers and thermoplastic filaments or from fibers which are coated with thermoplastic powder. However, the only method of forming a racquet preform which is described in the patent is braiding the tow to form a braided sleeve.
Thermoplastic unitapes are available for making composite products. Raw materials in this form are stiff, flat, and cannot be formed without the application of heat. Composite tubes and pipes made with narrow thermoplastic unitapes are produced by the addition of an intense heat source, typically an open torch flame or laser beam, and are filament wound at high speeds to avoid overheating a local area. A thermoplastic composite tube made by this method is not a flexible preform and cannot be formed to a game racquet shape for insertion into the mold.
Filament winding is a well known process for products from filament material such as graphite fiber, glass fiber, etc. In the filament winding process, the filament material is wound around a rotating mandrel to form a tube. The filament material can be in the form of a tow, which is formed from a plurality of filaments or continuous fibers. The filament material is coated with resin before or after winding. The wound tube is flexible and is placed in a mold having the shape of the end product, and the mold is heated to cure the resin. An alternative process is to filament wind reinforcing fibers into a preform shape and load the preform into a mold for resin transfer molding (RTM) or reaction injection molding (RIM). The RTM process is used only for thermoset resins. The RIM process has been explored by injecting braided reinforcements with polyamide monomers to form a tennis racquet frame.
Filament wound pans are normally manufactured by winding discrete layers of material on a mandrel, using a helical or geodesic path. Each layer is made up of a 2-ply balanced laminate formed from a multicircuit winding pattern. The number of circuits required for each layer is calculated from the bandwidth of the material and the wind angle, so that the pattern closes and a constant thickness is achieved in the layer. Different layers may be wound at different angles, but each layer typically consists of a +.alpha..degree. angle ply and a -.alpha..degree. angle ply with respect to the longitudinal axis of the rotating mandrel, resulting from the reciprocating traverse of the carriage in the winding machine.
Conventional filament winding produces discrete layers or lamina of fibers. The layers have a tendency to move relative to each other when the wound article is stressed, during removal from the mandrel and loading of the wound tube in the mold. Also, the physical properties of the article are often limited by the shear properties between layers, called interlaminar shear.
Tennis racquets have been produced by conventional filament winding. For example, EPO patent publication No. 0 470 896 describes forming a frame for a tennis racquet by winding fibers of glass, carbon, or other materials. U.S. Pat. No. 4,871,491 describes filament winding a preform from a tow of carbon fibers and a tow of thermoplastic fibers which are intermixed. Column 13, lines 17-18 refer to using the fiber blend to form racquet frames, but no specific procedure for forming racquet frames is described. Column 13, lines 20-57 refer to filament winding applications. However, no specific preforms are described, and the patent states that the carbon fiber/thermoplastic fiber tow is heated as soon as or soon after it meets the mandrel for melting or fusing the thermoplastic.
The following definitions are used herein:
CIRCUIT: One complete traverse of the fiber feed mechanism of a filament winding machine. PA1 INTERSPERSE: To sequence wind circuits from two or more patterns in an arbitrary manner as selected by the designer. Each circuit is applied individually such that any combination and sequence of wind angles from the patterns may by applied. No discrete layers or lamina exist. PA1 TOW: Untwisted bundle of continuous filaments PA1 PLY: A single pass on a filament winding machine in which fiber is applied in one direction to the axis of the mandrel. PA1 A LAYER is typically formed from a series of winding circuits resulting in a closed pattern of 2-plies; one of +.alpha..degree. and the other of -.alpha..degree.. PA1 LAMINA: A single ply or layer in a laminate made up of a series of layers.